Slice the cucumbers into thin rounds or half-moons. If using English cucumbers, you can leave the skin on.
Thinly slice the red onion into rings or half-rings. Place in a small bowl and cover with cold water. Let soak for 5 minutes, then drain well.
In a small bowl, whisk together lemon juice, lemon zest, olive oil, salt, pepper, and sugar until combined.
In a large bowl, combine the sliced cucumbers and drained red onion.
Pour the dressing over the vegetables and toss gently to coat.
Let the salad sit for 5 minutes at room temperature to allow flavors to meld. Garnish with fresh dill or parsley if desired. Serve immediately or refrigerate.
